This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-014196, filed Jan. 23, 2001, the entire content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a rotating pile for undergrounding with an improved supporting force.
2. Description of the Related Art
In constructing a building, a foundation work is performed in general in the construction process. Where the ground is weak, piles are driven into the ground in the foundation work before placing the concrete foundation.
Various kinds of piles are used for the foundation work. For example, a rotating pile for undergrounding is disclosed in Japanese Patent No. 2893443. The rotating pile for undergrounding disclosed in this Japanese Patent is constructed as shown in FIGS. 16 to 18. FIG. 16 is a perspective view showing the conventional rotating pile 1 for undergrounding. FIG. 17 is a plan view, as viewed from one end side, of the rotating pile 1 shown in FIG. 16. FIG. 18 shows the rotating pile 1 shown in FIG. 16, which is driven into the ground.
The rotating pile 1 includes a pile-head pipe 2 having a pipe wall 21, a longitudinal center line L, one end portion 22, and a pipe body 24. A spindle section 3 and two blades 4 are formed in the end portion 22. The two blades 4 are separated from each other at the longitudinal center line L. The spindle section 3 and the blades 4 are formed by crushing with a forming apparatus from both sides in a radial direction perpendicular to the longitudinal center line L. That is, the spindle section 3 and the blades 4 are formed by a press-forming (forging) with the forming apparatus.
The spindle section 3 is flared from the end portion 22 toward the pipe body 24 along the longitudinal center line 1.
Two blades 4 extend from the spindle portion 3 along the longitudinal center line L and most protrude in the direction of the center line L in the vicinity of the center line L. To be more specific, when viewed in a direction perpendicular to the longitudinal center line L, the tips of the two blades along the center line L are formed to be V-shaped. The two blades 4 are twisted by a predetermined angle xcex1 (see FIG. 17) in the rotating direction about the longitudinal center line L. The other end portion opposite to the one end portion 22 is connected to a pile-driving apparatus (not shown).
As shown in FIG. 18, the rotating pile 1 is driven by the pile-driving apparatus so as to be driven into the ground 5 such that the end portion 22 of the rotating pile 1 is driven first into the ground 5 while the rotating pile 1 is being rotated about the center line L. Since the rotating pile 1 is provided with the two blades 4, a hole 6 is formed in the ground while removing the soil 7. Therefore, the rotating pile 1 can be driven into the ground 5 in a short time. Also, the blades 4 are twisted as described above. Therefore, the soil 7 removed by the blades 4 moves from the circumferential direction of the pile-head pipe 2. Also, the spindle portion 3 is formed such that the contour of the spindle portion 3 is gradually increased from the end portion 22. As a result, the soil 7 moved in the circumferential direction is pressed against the inner wall of the hole 6 by the side walls of the spindle portion 3 and the pile-head pipe 2. What should also be noted is that, since the soil 7 is pressed against the inner wall of the hole 6, the soil 7 is scarcely discharged from within the hole 6 to the outside. It follows that the rotating pile 1 for undergrounding can be driven into the ground without requiring the troublesome disposal of the soil discharged to the outside.
As described above, the end portion 22 is formed by the press-forming so as to form integral the pipe body 24 and the blades 4. As a result, the rotating pile 1 can be manufactured at a low cost. Also, in the conventional rotating pile 1 shown in the drawing, the spindle portion 3 and the blades 4 can be formed after the pile-head pipe 2 is cut into a desired length.
In the conventional rotating pile 1 for undergrounding, the two blades 4 are formed simultaneously by crushing the end portion 22 from both sides in the radial direction as described previously. Also, the rotating pile 1 for undergrounding has a tip in the radial direction of the pile-head pipe. The tip has a distance from the longitudinal center line, which is about 1.4 times as much as the radius of the pile-head pipe 2. In other words, the blades 4 protrude greatly to the outside from the outer circumference of the pile-head pipe 2. As a result, the blades 4 protruding to the outside soften the soil 7 outside the outer circumference of the pile-head pipe 2 when the rotating pile 1 is driven into the ground. Incidentally, the supporting force of the pile of this type is equal in general to the resultant of forces between the bottom surface-supporting force proportional to the cross sectional area of the bottom of the pile and the frictional force between the side surface of the pile and the soil 7 in contact with the side surface of the pile. In the conventional rotating pile 1 for undergrounding, the surrounding soil 7 is softened over a wide range so as to lower the frictional force. Thus, in the conventional rotating pile 1, the entire supporting force is decreased. In other words, the supporting force of the rotating pile 1 is lowered after the rotating pile 1 is driven into the ground.
Where the rotating pile 1 is driven into the hard ground, those portions of the blades 4 which greatly protrude from the circumference of the pile-head pipe 2 may be bent. In this case, it is difficult to drive smoothly the rotating pile 1 for undergrounding into the ground.
Also, the blades 4 have a V-shaped tip like an auger along the center line L as described previously. If the tip of the blades 4 strikes against a hard material such as a stone, the axis of rotation of the blades 4 is deviated. Therefore, it is possible for the rotating pile 1 to fail to be driven vertically downward. Further, the blades 4 are twisted as described previously, with the result that the forging die for preparing the blades 4 is rendered complex in construction and expensive.
The present invention, which has been achieved in view of the situation described above, is intended to provide a rotating pile for undergrounding which can be manufactured easily and which has a high supporting force.
A rotating pile for undergrounding according to a first aspect of the present invention comprises a pile-head pipe. The pile-head pipe has a longitudinal center line, a pipe wall, one end portion, and the other end portion on the opposite side of the one end portion. Three or four recessed portions in which the pipe wall is recessed toward the longitudinal center line are formed in the one end portion of the pile-head pipe. Also, the blades defined by the pipe wall are formed between the adjacent recessed portions. Naturally, the number of recessed portions is equal to the number of blades. Each of the recessed portions is inclined from the circumferential surface of the pile-head pipe toward the longitudinal center line. Each blade is defined between the adjacent recessed portions. The pipe wall defining one of the adjacent recessed portions contacts with the pipe wall defining the other recessed portion. The one end of the pile-head pipe is thereby substantially closed. Also, the blades extend radially outward from the longitudinal center line. Further, the blades gradually expand in the circumferential direction of the pile-head pipe from its top end of toward the base end portion.
According to a second aspect of the present invention, there is provided a rotating pile for undergrounding, comprising a pile-head pipe and a pile-head-supporting pipe. The pile-head pipe includes a longitudinal center line, a pipe wall, one end portion, and the other end portion opposite to the one end portion referred to above. Three or four recessed portions in which the pipe wall is recessed toward the longitudinal center line are formed in the one end portion noted above. Also, in the one end portion, the blades defined by the pipe wall are formed between the adjacent recessed portions. Naturally, the number of recessed portions is equal to the number of blades. Joining-portions expanded in a flared fashion are formed in the other end portion of pile-head pipe. Each of the recessed portions is inclined from the circumferential surface of the pile-head pipe toward the longitudinal center line. Each blade is defined between the adjacent recessed portions. The pipe wall defining one of the adjacent recessed portions contact with the pipe wall defining the other recessed portion so as to cause the one end of the pile-head pipe to be substantially closed. Also, the blades extend radially outward from the longitudinal center line. Further, the blades gradually expand in the circumferential direction of the pile-head pipe from its top end of toward the base end portion. The pile-head-supporting pipe is joined to the pile-head pipe in the joining-portion.
Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention. The objects and advantages of the present invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.